1. Field of the Invention
This invention relates to shielding for ion implantation chambers to reduce particle contamination within the target chamber.
2. Brief Description of the Prior Art
A standard target chamber for implantation of ions into a semiconductor wafer generally includes an aperture in a side wall thereof for entry of the ion beam into the target chamber. Within the target chamber, a reciprocatable shaft, which is vertically reciprocatable, extends through the floor of the target chamber and supports and partially positions the wafer and other wafer securing and positioning mechanisms. The positioning mechanisms include a platen to which the wafer into which ions are to be implanted is secured. The platen is secured to a motorized apparatus which rotates the face of the platen holding the wafer up to ninety degrees from an upwardly facing direction to a direction facing the aperture through which the ion beam enters the target chamber. A wafer is loaded onto the platen while the wafer holding face of the platen is directed upwardly. After loading, the wafer is rotated to face the ion beam by operation of the motorized apparatus. The motorized apparatus is secured to a motor which rotates the motorized apparatus and platen at a 45 degree angle from the horizontal plane. The entire apparatus within the target chamber rests upon the shaft which is movable in a vertical direction to position the wafer in a vertical direction relative to the ion beam. The wafer can be rotated 360 degrees about its axis.
During ion implantation, the ionized material not only lands on the target, but some also condenses on the entire assembly within the target chamber including the chamber walls and the shaft. As the shaft moves downward in the course of its reciprocating movement, it rubs on a seal disposed in the bottom of the chamber and surrounding the shaft. This rubbing action causes the now non-ionic material condensed on the shaft, generally but not limited to arsenic, to be removed from the shaft and build up as particles, such as particles of arsenic, in the region of the seal on the floor of the target chamber. The build-up of particles of condensed material near the seal is a major cause of uncleanliness within the target chamber since the particles can ultimately land on wafers being processed within the chamber and add to the particle count on the wafer or cause other problems, thereby providing an ion implantation level other than that desired or in other ways reducing production yields. It is therefore readily apparent that yields will be improved by preventing the particles from landing on wafers being processed within the target chamber.